The invention relates to a method of manufacturing a combustor for a gas turbine engine having an air permeable open cell metal foam core bounded by perforated thin metal or ceramic walls inwardly and outwardly.
The invention includes manufacturing a composite wall having an open cell metal foam core layer bonded to an inner and outer layer of metal or ceramic, that can be used for constructing the walls of a high temperature low cost combustor chamber for a gas turbine engine.
A common prior art annular combustor is constructed of large sections that have thin metal walls that are machined down in thickness from a single forging as for example shown in U.S. Pat. No. 6,079,199 to McCaldon et al. issued Jun. 27, 2000. Large sections of the combustor are machined from a single forging or the entire combustor shell is constructed from several individually machined panels, each from a separate forging and thereafter precisely welded together.
However, this method for creating a combustor shell is less than optimum due to the inherent limitations of fitting, welding and machining large components to the required finished tolerances. In order to economically produce a combustor wall, sections may be left relatively thick in cross section to reduce the amount of machining time required and also to reduce the difficulty involved in machining very thin shells having a large diameter. As a result, therefore, prior art combustors can be very heavy with mechanical strength that far exceeds the requirements of the engine and the requirements of the combustor as a pressure vessel. Joints between panels are left relatively thick in order to permit the drilling of a large number of small cooling holes that are required to develop a cooling protective air film in a down stream combustor section.
The metal structures are expensive, difficult to machine from tough high strength expensive materials, and may still require a coating of a ceramic thermal barrier on the inner surface to protect the metal. The complexity of the surface features and a large number of cooling holes make application of the spray ceramic coating a time consuming and expensive proposition, due to the amount of preparation work in masking over openings to avoid covering the cooling openings or grooves to maintain their function. Although modern fabrication techniques employing computer control have somewhat mitigated manufacturing costs, the modern combustor is still an expensive structure to produce.
The role of the combustor is to serve as a heat shield protecting the walls of the pressure vessel, which surrounds the combustor and contains compressed air from the compressor. Combustion gases are produced from ignition of the fuel and air mixture, and the combustor also serves to physically duct the combustion gases and protect the adjacent portions of the engine from the extreme heat of the combustion gases. The combustor also meters the compressed air flowing into the combustor in a specific proportion creating a fuel/air mixture that allows the formation of a stable flame zone within the combustor. If airflow was not partitioned and metered within the combustor, the flame would be difficult to establish and maintain, thereby leading to engine performance that is extremely unreliable.
However, the combustor in practice is a little more than a gas flow baffle that separates gases of different temperatures. It meters the flow of compressed air into the combustion zone and structurally resists a modest pressure drop across it""s surface as air enters cooling holes and metering holes. The load imposed by this pressure differential acting on the combustor walls is relatively low and a very thin walled section could easily support the pressure difference. The greatest stress on the combustor walls results from large temperature gradients generated by the non-homogeneous gas temperatures within the combustor that result in differential thermal stresses, and are dependent on the efficiency of air/fuel mixing. The higher the temperature gradients within the combustor, the higher the thermal stresses that the combustor must resist. The wall thickness in a homogeneous material such as nickel alloy also aggravates the gradient and stresses.
It is an object of the present invention to produce an improved combustor for a gas turbine engine that can be manufactured more cheaply and offers better performance.
It is a further object of the invention to provide a method for manufacturing an improved combustor
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.
The invention provides a method of manufacturing a composite wall for a gas turbine engine combustor having an open cell metal foam core layer bonded to an inner and outer cladding layer of metal or ceramic.
A core substrate of open cell gas permeable foam is created in a selected geometry, for example of molded polyurethane foam rubber. The substrate is easily molded and can be thermally converted to a relatively rigid but brittle carbon structure that may be easily machined. The open cell carbon foam substrate is then impregnated with metal vapour and a porous layer of metal is deposited on exposed internal and external surfaces of the substrate thereby forming the open cell metal foam core through metal vapour deposition. Formation of nickel-aluminum foam structures are described in U.S. Pat. No. 5,951,791 to Bell et al, which is incorporated by reference herein.
Thin inner and outer cladding layers are formed upon the metal foam core through spray application of metal or ceramic cladding materials. Masking of the metal foam core before spraying results in formation of ports or slots for gas flow through the composite wall for cooling, air film formation, filtering or other purposes. The impregnating step may include exposing the substrate to nickel vapour and thereafter coating the nickel metal foam core with aluminium through metal vapour deposition that can further be reacted to form a nickel aluminide metal foam core.